There is substantial need throughout the world for technologies that are capable of producing safe and effective recycled products from various types of wastes, including "hazardous" wastes containing heavy metals. The most desirable method of recycling employs wastes as raw materials in the production of other safely usable products. The "hazardous" constituents of these products are often useful and valuable, yet, they are not fully exploited. Known recovery processes which attempt to recover certain elements from the waste stream, typically leave a substantial amount of residual slag or similar residual waste materials. These residuals typically contain "hazardous" components harmful to the environment.
The present invention is particularly well suited for recovering and reusing inorganic wastes such as emission control dusts or sludges. These wastes typically contain economically valuable levels of ingredients for making glass. They also typically contain organics, certain less valuable inorganic ingredients, and certain "hazardous" ingredients, such as lead, cadmium, chromium oxides or other heavy metals and/or compounds containing heavy metals.
An increasing measure of environmental concern throughout the world has grown from professional and public awareness of the increasing hazard of landfilling heavy metals and other inorganic wastes. Inorganic wastes are often hazardous due to their ability to react with acids and release soluble heavy metal compounds into the environment. This has focused attention on reducing landfilling and on regulations that prevent leachable toxic materials from being disposed of in landfills. The extent of groundwater pollution from leaching of heavy metals and other inorganic toxins is only now being understood.
As a result of groundwater pollution, federal law mandates that inorganic hazardous wastes now be "stabilized" prior to being landfilled. Stabilizing of inorganic wastes which renders the wastes inert or unreactive is a costly process and not always available. This has or will increased the cost of landfilling and, accordingly, the operating costs of the generators of the waste.
The cost of stabilization and landfilling essentially purchases a volume of space in a landfill. While the landfill will eventually become full and be "closed," the waste, stabilized or otherwise, remains in the ground exposed to other reactive elements. Inherent in any landfill, operating or closed, is the potential liability imposed in the United States by the Comprehensive Environmental Resources Compensation and Liability Act and in other countries, by similar laws.
Under these laws and the implementing regulations, corporations which generate waste remain liable for any damage which that waste may do to the environment. Further, the Resource Conservation and Recovery Act extends liability to waste generators which place wastes in landfills that subsequently leach or become contaminated, for the clean-up of those landfills. The waste generators' exposure is potentially huge, and does not end after a specific period of time. The potential liability extends in perpetuity and follows the waste generator and any subsequent entity related to the waste generator. This endless liability is another cost to corporations and, in certain cases, actually threatens a waste generator's continued existence.
The typical inorganic waste streams that contain these heavy metals are sludges from various processes or from waste water treatment; emission control dusts from high temperature industrial processes; fly ash from incineration of industrial and residential wastes; and certain other process-specific effluents. Examples of these are the aluminum industry's spent pot liner; refractory wastes from smelting, melting or refining furnaces; various types of slags and precipitants related to metal recovery operations from waste streams and certain glass wastes from producing television and cathode ray tubes.
The existing technology for dealing with these wastes is to use reduction or precipitation processes to recover metals from emission control dusts and sludges mostly from the steel industry and the plating industry. Emission control dusts are subjected to a reducing process in kilns to reduce chromium, zinc, and nickel from their oxides. In such cases, a fee is charged for processing and the recovered metals are sold back to the generator for reuse. Certain of the furnace slags, particularly in the aluminum industry, are processed to recover alumina, aluminum, and certain fluxing salts in a waste minimization program.
Methods of waste stabilization include: incorporating the hazardous wastes into cement; various types of chemical treatments; and vitrification, i.e., incorporating the hazardous waste into a glassy material. The principal danger from inorganic wastes is from heavy metal oxides and certain other oxides which may be considered "hazardous". Heavy, metal oxides readily react when exposed to strong acids or alkalines to produce soluble compounds. Vitrifying the metal oxide in a glass substantially reduces the oxides' solubility in acid. The incorporation of these materials in glass is one known long-term method of "stabilization".
This method, however, has proven to be extremely expensive for several reasons. First, substantial amounts of energy are typically required to melt the materials. Care must also be taken in batching the raw material to ensure that an amorphous glass is formed at the most insoluble phase of the glass material. Both factors significantly increase the cost of vitrification relative to rival technologies. Although vitrification is a highly desirable method of waste treatment, it is rarely used commercially because of its high cost. It has, however, frequently been utilized in laboratory research. Except in isolated circumstances, the process has never been put into commercial practice. Patents have been issued disclosing vitrification processes, however those processes each have limitations that are addressed by the instant invention.
One such patent is U.S. Pat. No. 5,009,511 to Sarko, et al for "INORGANIC RECYCLING PROCESS", issued Apr. 23, 1991. The Sarko, et al reference teaches a mobile system for fixing hazardous wastes in a silicate matrix for subsequent disposal. Although this reference teaches a vitrification process for disposal of hazardous wastes, it is directed to stabilizing those wastes in a silicate matrix. Further, while this type of material is acceptable for use in stabilizing wastes, it is inappropriate for fabrication of a high hardness, abrasive material. Moreover, Sarko, et al does not disclose a material having the desirable characteristic of an alumina content in the range of 15 to 30%. contemplated by this reference.
Another example is U.S. Pat. No. 5,002,897 to Balcar, et al. for "METHOD FOR HAZARDOUS WASTE REMOVAL AND NEUTRALIZATION," which discloses a method for disposing of hazardous waste from secondary aluminum smelting in a neutralized amorphous glass product. The waste gas stream from the aluminum smelting process is directed towards a baghouse dust collector which comprises a series of fibrous filters to remove residues from the waste stream. The residue forms a coating on the filters, which is periodically removed and discarded as hazardous waste. The hazardous wastes which include metal oxides, are combined with glass dust and sodium silicate are melted to the molten state. These additives neutralize the wastes by dissolving the metal oxides in the molten glass to form a neutralized amorphous glass. The glass in turn helps prevent leaching of the metal oxides.
However, Balcar, et al does not describe a process which provides for the extraction of soluble salts via a hot water extraction process. Moreover, Balcar, et al is directed to collection, via bag-house filtration, and re-processing of hazardous waste products resulting from the aluminum smelting process. Additionally, Balcar, et al requires that powdered soda-lime glass be seeded into the flow of air-borne hazardous waste. The seeded soda-lime glass and hazardous waste is directed into a baghouse as part of the pre-coat material, which also includes lime, generally hydrated agricultural lime or calcium hydroxide. As the precoat of the baghouse, the soda-lime glass powder and the lime is intimately mixed with the effluents from the furnace or other heating device. This had the undesirable effect of increasing raw materials costs. Moreover, addition of soda-lime glass powder and lime renders the fabrication of high hardness, alumina containing materials relatively expensive, while reducing yield of usable end product.
Thus, a need exists for an environmentally acceptable, efficient, and cost-effective process for removing and neutralizing hazardous wastes, such as heavy metal oxides, from the waste stream and for reusing or recycling the wastes as raw materials in the production of safe and useful products. One problem that has threatened some attempts to resolve these concerns is the lack of appropriate reasonable uses for recycled products incorporating heavy metal oxides. By virtue of its hardness and toughness, amorphous glass is useful as an abrasive. In glass, lead oxide in small quantities can serve as a toughening agent.
There are essentially four market segments for abrasives. First, bonded abrasives are manufactured from abrasive particles which are normally joined by some form of organic binding agent. For example, bonded abrasive may form grinding and cutting wheels. Another form of bonded abrasives are tumbling abrasives, which are typically used in fine metal finishing.
The second general market for abrasives is for so-called "coated abrasives." In these, the abrasive is bonded to a substrate. For example, the abrasive may be bonded to paper, to form sandpaper.
The third general market segment for abrasives is for "loose grain" abrasives, in which the abrasive particles are thrown at a surface at high speed, in a stream of compressed air or pressurized water. The abrasives attack the surfaces of the metal, removing coatings or scale and leaving a clean surface. While sand is often used as a loose grain abrasive, its use is now restricted in several countries. This is due to the potential of silica-based fine abrasive particles to cause lung damage. Other fine loose grain abrasives include glass beads and fused aluminum oxide. These latter two abrasives are typically used for removing scale, coatings, and for finishing aircraft engine parts or materials which must meet specifications for a smooth surface finish. They are called "fine" because they are fine or small particle. Standards exist for the cleaning of aircraft surfaces with such treatments.
Loose grain media are also used for shot peening, for transforming the stresses in the surfaces of metals from tensile to compressive. This is done mainly to increase the useful life of the metal or to increase its resistance to fatigue cracking. Fine loose grain abrasive cleaning and finishing and shot peening are extremely efficient, as compared to other alternate methods of smoothing surfaces.
The fourth general form of abrasive is polishing agents. These are typically hard materials of extremely fine particles ranging in size from 1 or 2 microns up to 25 or 30 microns. These are often used in connection with cleaning, typically in burnishing and polishing machinery for finishing metals, ceramics, and glass articles.
A common material used for all of these types of abrasives is fused aluminum oxide. Fused aluminum oxide is manufactured by heating alumina to extremely high temperatures, i.e., approximately 2200.degree.-2400.degree. C. typically in an electric arc furnace, to produce a mass of crystalline alumina. Crystalline alumina is denser than natural alumina and possesses greater strength. Fused aluminum oxide has a hardness between 8 and 9 on the MOH scale, or a Vickers hardness of approximately 2000. In contrast, crystalline quartz or sand has a hardness of only about 7 on the MOH scale. Common soda lime glass has a Vickers hardness of approximately 420 and quartz has a hardness of 590. Soda lime glass in beaded form is well known as a cleaning and finishing media, and it is also used for shot peening.
While crushed soda lime glass is inexpensive and may also be used as an abrasive, it has a severe disadvantage. Hydrolysis will occur between the particles if they become wet. This binds the particles together, causing clumping and, eventually hardening. Hydrolysis typically occurs between the sodium ions on the surface of the glass. Sodium oxide makes up approximately between 12-14% of a typical soda lime formulation. This is also true of glass beads. Glass beads, however, are easier to keep dry with desiccation due to their reduced surface area. However, glass beads are not suitable for the applications to which angular abrasives are normally put. This is because glass beads are spherical and thus clean by peening a surface underneath a coating, cracking the coating off. If a coating is bonded adhesively to a surface, glass beads do not work as well since they do not break the adhesive bond. Rather angular abrasive materials, such as fused aluminum oxide, which has cutting surfaces, cut into contaminants and coatings to remove them faster than glass beads.
Quartz suffers from another deficiency, which is generally considered more serious than hydroscopicity. When shattered, quartz fractures along the silicon-to-oxygen bonds, leaving unbonded silicon. This is known in the trade as "free silica". This unbonded silicon (SiO) reacts slowly at ambient temperatures with molecular oxygen (O.sub.2). Free silica, when it reacts with oxygen, forms silica (SiO.sub.2). Fine, breathable particles of free silica can combine quickly with nascent oxygen in the lungs of humans or animals and bind to the lung tissue. This hardens the lung tissue and reduces its oxygenation potential. Over time, this process results in an irreversible deterioration in lung function. This disease process has come to be known as "silicosis." As a result of the health risks of silica, the use of fine silica particles is outlawed or restricted by regulation in many countries.
Thus, a need exists for an abrasive particle in the hardness range between glass and fused aluminum oxide. This material should have a hardness of approximately 7 MOH. It should not be hydroscopic, allowing it to be separated by water sedimentation methods. It should also be environmentally acceptable.
The invention described herein addresses these needs by employing emission control dusts from the secondary aluminum industry, as well as other segments of the metals industry, as well as fly ash from industrial incinerators, to produce a glass abrasive product containing a high percentage of alumina, with a Vickers hardness of between approximately 620 and 680. None of the known approaches, prior to the present invention, are able to resolve all of these concerns in as cost-effective and reliable a manner as is the present invention.